The present invention relates to a semiconductor device having a compound semiconductor layer grown on a substrate with a buffer layer in between.
A compound semiconductor has an inherent property such as a direct transition property. Accordingly, a semiconductor device formed of the compound semiconductor is expected to have a high breakdown voltage and operate at a high frequency. Among the semiconductor devices, a high electron mobility transistor (HEMT), i.e., a type of a field effect transistor (FET), formed of a nitride-based compound semiconductor has been attracting an attention, and a variety of HEMTs have been proposed.
FIG. 15 is a cross sectional view showing a conventional HEMT 11 formed of a GaN-based compound semiconductor as the nitride-based compound semiconductor. In the conventional HEMT 11, a low temperature buffer layer 13 formed of GaN and grown at a low temperature, a buffer layer 14 formed of GaN, an electron drift layer 15 formed of GaN, and an electron supplying layer 16 formed of AlGaN are sequentially grown in this order on a substrate 12 formed of sapphire or the like, thereby forming a hetro-junction structure. A source electrode 17S, a gate electrode 17G, and a drain electrode 17D are disposed on the electron supplying layer 16. A contact layer (not shown) formed of n-GaN is formed between the source electrode 17S or the drain electrode 17D and the electron supplying layer 16 for reducing a contact resistance in between.
In the conventional HEMT 11, a two-dimensional electron gas layer 15a generated beneath the heterojunction interface between the electron drift layer 15 and the electron supplying layer 16 is used as carriers. When the source electrode 17S and the drain electrode 17D are operated, electrons supplied in the electron drift layer 15 move at a high speed through the two-dimensional electron gas layer 15a to the drain electrode 17D. In this case, a voltage applied to the gate electrode 17G is controlled and a thickness of a depletion layer beneath the gate electrode 17G is adjusted, so that the electrons moving from the source electrode 17S to the drain electrode 17G, namely a drain current, can be controlled.
In the semiconductor device such as the HEMT using the nitride-based compound semiconductor such as GaN, generally, a buffer layer needs to have a high resistance in order to restrain a leak current in the buffer layer. If the buffer layer does not have a high resistance in the HEMT 11 shown in FIG. 15, a leak current tends to flow in the buffer layer 14 or the low temperature buffer layer 13 even when the depletion layer beneath the gate electrode 17G is enlarged to turn off the drain current, thereby making it difficult to completely turn off the drain current. To this end, a conventional method has been proposed for increasing a resistance value of the buffer layer (refer to Patent References 1 and 2). Patent References 1 and 2 have disclosed a method of doping an impurity such as Zn, Mg or the like in the buffer layer formed of GaN to increase the resistance value thereof.    Patent References 1: Japanese patent publication No. 2002-057158    Patent References 2: Japanese patent publication No. 2003-197643
In the semiconductor device such as the HEMT having the buffer layer with the increased resistance by impurity doping, a current collapse becomes apparent, i.e., a phenomenon in which electrical characteristics related to an output current vary with time. The current collapse is considered to occur when a part of the doped impurity not activated is charged, thereby preventing the electrons in the two-dimensional electron gas layer from moving.
In the field effect transistor having an insulation gate such as an MISFET (Metal Insulator Semiconductor FET), an MOSFET (Metal Oxide Semiconductor FET), or the like, when the p-type impurity such as Zn, Mg, or the like is doped in the GaN-based compound semiconductor, it is possible to increase the resistance value of the buffer layer according to a density of the impurity, thereby increasing a breakdown voltage of the element. However, when the element has a higher breakdown voltage, a threshold voltage thereof tends to increase, thereby lowering controllability of the field effect transistor.
In view of the problems described above, an object of the present invention is to provide a semiconductor device capable of increasing a resistance of a buffer layer without deteriorating the current collapse, thereby reducing a leak current in the buffer layer. A further object of the present invention is to provide a semiconductor device capable of increasing voltage resistant without increasing a threshold voltage.